This invention relates to an apparatus and method for measuring the thickness of a wafer. More particularly, this invention relates to an apparatus and method for measuring the thickness of a wafer on an edge grinding or edge polishing machine.
As is known, edge grinding machines and edge polishing machines are typically constructed so that the peripheral edge of a wafer may be ground or polished during rotation of the wafer about a fixed axis. From time to time, it becomes necessary to measure the thickness of the wafer along the edge, for example to determine the degree of grinding/ polishing.
Heretofore, various types of measuring techniques have been employed for measuring the thickness of a wafer on a edge grinder in order to determine if the xe2x80x9cdesired degree of grindingxe2x80x9d or polishing has been achieved. For example, some techniques have employed devices which rely upon contact with the wafer while other techniques employ non-contact devices.
The contact type of measurement is typically performed by holding the bottom surface of the wafer to be measured against a reference plane and then moving a small spherical surface or stylus into contact with the top surface of the wafer. In such cases, a linear transducer generally follows the position of the stylus and uses the contact position to calculate the wafer thickness. However, one problem with this technique is that only a single point contact is used to obtain a measurement of the overall thickness of the wafer. Should the wafer have a varying thickness at different points, a single point contact reading may well be inaccurate. Furthermore, small areas of contact can scratch, fracture or chip the wafer and thus are not cost-effective from this standpoint.
The non-contact type of measurement typically use capacitance sensors, optical sensors and air followers to obtain a measurement of thickness. The capacitive and optical sensors usually take their measurements from a fixed location above the wafer. On the other hand, the air followers typically measure the displacement of a following head of the edge grinding machine as the head moves to the wafer. These techniques may employ one or two sensors. In the case of two sensors, one is located above the wafer and the other directly below. In the case of using only one sensor, the bottom surface of the wafer is held against a reference plane and the distance to the top surface of the wafer is measured.
Capacitive sensors, however, are not able to accurately measure the thickness of wafers which vary in resistivity. Wafers made of glass, various compound semi-conductors and wafers with devices on them are examples of varying resistivity that the capacitive sensors have problems reading.
Optical sensors have problems reading wafers that are transparent or reflect their emitted light beam.
Air follower techniques can measure all types of wafers but require more maintenance and typically are less accurate than the capacitive or optical sensors.
Accordingly, it is an object of the invention to provide a relatively simple apparatus and method for measuring the thickness of a wafer on an edge grinding machine or an edge polishing machine.
It is another object of the invention to provide a simple a reliable device for measuring wafer thickness.
Briefly, the invention is used on an apparatus for grinding an edge of a wafer or for polishing a wafer which has a pair of relatively movable vacuum chucks for engaging the wafer therebetween. The invention employs at least one sensor for measuring the distance between a movable one of the chucks and a fixed reference surface. The sensor is used to establish a standard measurement value corresponding to the distance between the movable chuck and the fixed reference surface when the movable chuck is in engagement with the other chuck without a wafer therebetween and to establish an actual measurement value for the distance between the movable chuck and the fixed reference surface when the movable chuck is in engagement with the wafer.
A calculator is also provided for determining a difference between the actual measurement value and a standard measurement value and displaying this difference as the thickness of the wafer.
The apparatus is constructed with means for selectively moving the movable chuck away from and towards the other of the chucks and means for selectively rotating the other chuck whereby the wafer can be rotated into different selective positions relative to the movable chuck in order to obtain multiple measurements of the thickness of the wafer.
The invention also provides a method measuring the thickness of a wafer while on a grinding or polishing machine employing a pair of chucks for contacting and holding the center of a wafer therebetween. In accordance with the method, the distance between one of the chucks when in engagement with the other chuck without a wafer therebetween is measured relative to a fixed reference surface in order to establish a standard measurement value. Thereafter, a wafer is placed between the chucks and the distance is again measured between the first chuck and the fixed reference surface in order to obtain an actual measurement value. A difference is then calculated between the actual measurement value and the standard measurement value and used as the thickness of the wafer.
The apparatus and method may be used to measure the average center thickness of all types of wafers. Material properties that effect optical or capacitive sensors are of no consequence.
The apparatus and method may employ one sensor or multiple sensors to measure the relative the displacements between the chucks. In either case, multiple measurements may be taken and an average of two or more thickness measurements may be made.
By rotating the wafer to obtain measurements at different points and averaging the thickness measurements compensates for any taper in the wafer and/or any wobble in the rotatable chuck on which the wafer is placed.